Although it has a relatively steep learning curve, JOSM is used by experienced editors thanks to its plugins and stability. There are other editors for OpenStreetMap data, like iD or Potlatch 2. JOSM is a feature-rich editor with an interface which can seem complex at first. Have a look at the comparison of editors if you're still unsure which editor to choose.

Usage

JOSM screenshot, editing in Cleveland, Ohio, USA

JOSM Guide

The JOSM/Guide will take you through getting started, downloading some data, basic editing and more advanced editing features. It is intended for beginners to read the guide from the beginning. This is a descriptive guide providing a simple introduction. Some details are intentionally left out.

JOSM Help

The JOSM Help manual should provide more exhaustive in-depth explanations of each JOSM feature. It is intended to be used as a reference manual, rather than reading from end-to-end. It is also accessible by pressing F1 within the application (context sensitive help)